An optoelectronic circuit, mounted on a printed circuit board to form an optoelectronic module, can be used in a small instrument, such as a wireless optical computer mouse or a pointing device or any other small instrument. For an application in a conventional RF wireless optical mouse, the light source used can be a light emitting diode. This LED diode must be placed on the printed circuit board at a certain distance from the photoreceptor in order to be able to emit a light beam at a certain angle relative to the work surface on which the optical mouse is placed. The light beam from the LED diode must thus be sufficiently low angled to be able to work with the work surface speckle and to reflect part of the light in the direction of the photoreceptor to detect a movement of the mouse.
Since the LED diode has to be mounted at a location on the printed circuit board that is sufficiently far from the location of the photoreceptor and the photosensitive area thereof, for example at a distance of the order of 10 mm, the optoelectronic module comprising said photoreceptor with its photosensitive area cannot be miniaturised beyond a certain limit. Moreover, a transparent unit with at least one lens must be provided on the diode for light transmission and on the photoreceptor for light reception. This thus makes the optoelectronic module relatively voluminous. If this diode and the photoreceptor are mounted on a flexible printed circuit board, certain parts must be further rigidified to prevent any shift between the light beam emitted by the diode and the reception part of the photoreceptor.
It should also be noted that if the LED diode generates a relatively low angled light beam, a large part of the light produced by the LED diode is wasted. Consequently, a lot of electrical and luminous energy is also wasted with significant noise. Moreover, with a conventional optoelectronic module for a wireless optical mouse, there is great sensitivity to the type of surfaces on which the optical mouse is placed, which is a drawback. It is preferable therefore to be able to work on a so-called Lambert surface for good light reception. Even with all the improvements that could be made, the electrical energy consumption remains relatively high, which can be detrimental to the lifetime of the two operating battery cells of this type of mouse.
In order to avoid the problems encountered with the use of an LED diode, one could imagine using a laser diode as for this invention. With a laser diode, it is possible to take advantage of the coherence of the laser beam that is emitted and reflected onto the work surface. The beam is no longer required to incline steeply in order to detect the movement of the optical mouse. Depending upon the path of the laser beam prior to reflection onto the work surface, there may be constructive or destructive interference between the emitted and reflected laser beam. This allows the optical mouse movement to be detected given that the depth of the work surface speckles may be greater than half of the wavelength of the emitted laser beam.
Thus, the laser diode and the photoreceptor of the optoelectronic circuit could be mounted on a printed circuit board and encapsulated in a single compact case of the optoelectronic module as shown in WO Patent Application No. 2006/042072. Generally, one aperture is provided in the module for emitting light towards the exterior and one aperture for receiving the light reflected in particular onto a surface. A lens unit for directing the emitted and received light is fixed onto the apertures of the module. This transparent unit blocks the apertures in order also to protect the laser diode and the photoreceptor from dust and environmental conditions.
Even when the laser diode and photoreceptor of the optoelectronic circuit are arranged in a single case of a conventional compact module, the dimensions of the module with the optoelectronic circuit cannot be reduced sufficiently. This constitutes a drawback for the various known modules with an optoelectronic circuit. Moreover, the production costs of such optoelectronic circuits and the resulting optoelectronic modules cannot be greatly reduced.